1. Field of the Invention
Exemplary embodiments of the present invention relate to methods of forming a metal oxide. More particularly, exemplary embodiments of the present invention relate to methods of forming a metal oxide using an atomic layer deposition process.
2. Description of the Related Art
As the degree of integration of a memory cell of a DRAM device increases, the memory cell occupies a reduced area on a semiconductor substrate.
Accordingly, the trend in the art requires that the capacitor of the DRAM device have improved capacitance. The capacitance of the capacitor may be expressed in accordance with the following equation 1.C=∈A/d  equation 1
In the above equation 1, “C” represents the capacitance of the capacitor, “∈” represents a dielectric constant of a dielectric layer, “A” represents a surface area of the capacitor, and “d” represents a distance between electrodes positioned on each side of the dielectric layer.
As shown in equation 1, the capacitance of the capacitor is proportional to the dielectric constant and the surface area of the capacitor, and is in inverse proportion to the distance between the electrodes.
To increase the capacitance of the capacitor using a conventional dielectric material such as silicon oxide or silicon nitride, a cylindrical capacitor or a fin-shaped capacitor has been developed to increase the surface area of the capacitor. However, applying the cylindrical capacitor or fin-shaped capacitor to the semiconductor device is difficult in practice because the process for forming the capacitor is complicated.
Use of high dielectric material such as Al2O3, Ta2O5, Nb2O5, ZrO2, and TiO2 to the dielectric layer of a capacitor has been proposed. The high dielectric material has a dielectric constant of about 10 to 114, which is about 2.5 to about 30 times greater than that of the conventional dielectric material such as silicon oxide (dielectric constant: 3.9).
Generally, the dielectric layer may be formed by a conventional chemical vapor deposition (CVD) process such as a low-pressure chemical vapor deposition (LPCVD) process or a plasma enhanced chemical vapor deposition (PECVD) process. Since the conventional CVD process is carried out at a substantially higher temperature, a layer formed through a conventional CVD process may have a relatively high content of impurities such as hydrogen, and also may have poor step coverage.
Considering the above-mentioned problems, an atomic layer deposition (ALD) process has been developed because a layer of a semiconductor device having good step coverage may be formed at a relatively low temperature.
A material that can be used as a precursor in the atomic layer deposition process must have some properties as follows. First, the material has a high saturation vapor pressure at a relatively low temperature and is chemically and thermally stable. In addition, when the material includes a metal and a ligand bonded to the metal, the ligand may rapidly separate from the metal. Further, the material is in a liquid-phase at a room temperature, and is nontoxic. Still further, the precursor must rapidly deposit on a substrate.
Alkyl metal compounds, metal alkoxides, metal halides, and β-diketonates are conventionally used as the precursor in the atomic layer deposition process. However, some alkyl metal compounds, such as Pb(C2H5)4, are toxic and explosive. Also, since metal alkoxides are sensitive to moisture, metals of the metal alkoxides are easily reacted with a hydrogen or a hydroxyl group to form a layer including impurities such as metal hydroxide. Further, β-diketonates are relatively expensive, have a low saturation vapor pressure, and are in a solid-phase at room temperature (J. of the European Ceramic Society, 19(1999), 1431-1434). Fluoro β-diketonates, such as hexafluoropentanedionate and heptafluorodimethyloctanedionate, may be used as the precursor in the atomic layer deposition process. However, fluoro β-diketonates may not actively react with a reaction gas so that the ligand is not easily separated from the metal, and the deposition rate is relative low due to the high molecular weight thereof.